I suppose there's a much more limited scope for innovation in 20 seconds of audio, and the "that bloody advert again" effect is probably just as effective[1] as a genuinely funny comedy sketch or a particularly insidious earworm.
Classic FM in particular suffers (well, I assume it still does - it's been a while) from the mental gear crunch effect of going from a piece of peaceful music to someone reading out financial small print as quickly as possible.
That said, there's something strangely pleasing about super-local low budget radio adverts. Probably because it hasn't changed since the 80s, like those slides you used to get in cinemas advertising the local curry house etc.
(Is Radio Jackie still going? I listened to that for a while, back in the days when streaming was new and exciting, and everyone else was playing the same music as Crapital FM[2]. There was something strangely exciting about listening to a local radio station for completely the wrong place, too.)
[1] Advertising effectiveness tends to be measured by how well the brand scores in "name n makes of $thing" surveys, which doesn't always translate to sales.
[2] They probably still are.